1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image which renders an electrostatic image formed on photoconductor surface visible, a developer containing the same, an image-forming apparatus using the same, and a toner container in which a color toner is contained.
2. Description of the Related Art
In the case of the dry method, an electrostatic image formed in electrophotography, electrostatic printing, electrostatic recording, and the like, is developed by a dry toner consisting mainly of a binder resin and a colorant and having a triboelectric charge given by a charger. Then the image is transferred and fixed on a sheet of paper. In order to obtain a high quality image, one of the most important technical challenges is the capability to faithfully develop a latent image formed on a photoconductor in any circumstances using a toner. Various attempts have been made to date.
Properties which are required for a developing agent to achieve high quality include storage property (blocking resistance), transport property, transfer property, charge property, fixing property, and the like.
As a general method to fix, or fuse, a dry toner image on a recording medium, it is common to use contact heat fusing in which a heated roller or belt is pressed against the toner image to fix it on the medium. This method is advantageous in that it has high thermal efficiency, is capable of high-speed fusing, and can give gloss and transparency to color toners. On the other hand, since the surface of the heat fusing member and a molten toner are in contact under a pressure and subsequently pulled apart, a portion of the toner image remains adhered to the surface of the fusing roller and then is applied on another image. Such phenomenon is called offset. To prevent the offset phenomenon, the method which has generally been employed is to form the surface of a fusing roller with silicone rubber or a fluorinated resin, which have excellent release property, and further apply releasing oil such as silicone oil on the surface of the fusing roller. This method is extremely effective in terms of preventing toner offset, but it requires a device to supply the release oil and a large fusing device, resulting in higher cost. Therefore, the trend for monochrome toners is to employ a method in which the viscoelasticity of a molten toner is raised by adjusting the molecular weight distribution of a binder resin or by using other ways so that the molten toner is not torn apart in the middle, and, moreover, a release agent such as wax is added in the toner to greatly reduce, if not eliminated, the use of release oil on the fusing roller.
However, in forming of a color image in full-color electrophotography, which generally uses three color toners of the three primary colors (yellow, magenta, and cyan) or four color toners by adding a black toner and laminate these toners to reproduce all the colors, a need exists for toners which are excellent in color reproducibility and which can give sharp full-color images. In order to obtain a sharp full-color image, it is required to reduce light scattering by making the surface of a fused toner image flat and smooth to a certain degree and also to lower the viscoelasticity when the toner is molten. Therefore, in forming a full-color image, an offset is more likely to occur than a no-gloss monochrome toner, thereby making it harder to realize an fusing device with no oil or little oil application. Additionally, when a release agent is added in a toner, the adhesiveness of the toner increases and the transfer property to transfer paper subsequently decline. Also, a problem arises as the release agent in the toner contaminates carrier particles and other members which are tribocharged (charged by friction) and reduces their charging property, thereby lowering the durability of the developer agent.
Regarding fusing methods, many cases employ heated roller fusing, which has a simple device configuration and easy handling. However, the heated roller fusing, which has conventionally been used by many, has the following problems:                (1) A certain period of time is required until the heating roller reaches a desired temperature (wait-time).        (2) It is necessary to maintain the heating roller at the optimum temperature in order to prevent bad fusing and offset phenomenon, which are caused by the fluctuation of the heating roller temperature due to the passing of recording materials or other external factors. To do so, the heat capacity of the heating roller or a heating member must be sufficiently large.        
In addition, for full-color toners, which are generally low in viscosity, there are other problems:                (3) Due to the curvature of the roller, offset occurs and the toner is taken up by the roller during paper output. Therefore, it is necessary to apply release oil and equip the system with an oil tank.        
In light of these problems, heated belt fusing has been suggested, and in addition, belt fusing techniques which applies no oil (oil-less method) or very little oil have been suggested. However, whereas it is common in full-color roller fusing to use a roller with a thick elastic member and apply a high pressure, in belt fusing, the applying pressure is small and its effect as a means to obtain high gloss is small.
There are several ways to achieve high gloss in belt fusing such as those described below.
Disclosed in Japan Patent Application Laid-Open (JP-A) No. 02-160250 is a method which defines an average particle diameter and an allowable amount of particles which are either too small or too large so as to reduce the roughness of a toner layer surface. However, in this case, a glossy image is not necessarily obtained even if the toner is adhered with little roughness on the recording material.
For increasing flatness and obtaining high gloss, in JP-A No. 11-125948, a method is described that defines the surface roughness of the image on an OHP sheet. However in this case, as the surface properties of an OHP sheet are different from those of paper, even if the surface roughness of the image on the OHP sheet is defined, offset and glossiness are not necessarily good depending on the surface properties of the paper.
Although glossy images have often been desired with full color images, demands for printer output are now large and therefore too much gloss is sometimes not desirable. Even more, a non-glossy image may be desired in some cases. Hence, it is required to be able to obtain images of different glossiness depending on the case. To address this need, JP-A No. 04-194967 proposes a fixing method wherein one can select between high gloss and no gloss with one fixing apparatus. However, conditions for obtaining high gloss are disadvantageous for offset, so depending on the conditions, offset tolerance may not be sufficient. Specifically, there is a problem that although offset does not occur in the solid area, it does in the halftone parts (fine offset).
Also, if the dependency of the gloss under conditional changes is too large, the gloss difference becomes so large that the glossiness is not stable under different conditions.
Various types of toner have been discussed in the related art. For example, JP-A No. 08-220808 proposes a toner using a linear polyester resin having a softening point of 90° C. to 120° C. and carnauba wax, JP-A No. 09-106105 proposes a toner containing a resin and wax which are compatible and have different softening points, JP-A No. 09-304964 proposes a toner specifying a melt viscosity of a polyester resin and wax, JP-A No. 10-293425 proposes a toner containing a polyester resin having a softening point of 90° C. to 120° C., rice wax, carnauba wax and a silicone oil, and JP-A No. 05-61242 proposes a wax-included polymer toner. Although all of these toners gave a suitable gloss, anti-offset properties were insufficient when release oil was not applied or was applied in only a small amount on the fixing roller, and the toners did not have very good transfer properties, durability, charge stability relative to humidity, or pulverizing properties.
In recent years, market demands for high-quality images are increasing, and as sufficiently high image qualities can no longer be obtained with a toner having a weight average particle diameter of 9 μm to 15 μm of the related art, a toner of still finer particle diameter is required. As the specific surface area increases the smaller particle diameter of the toner is, the powder fluidity of the parent colorant particles decreases, a large amount of external additives must be added as a surface treatment to confer fluidity. If it is attempted to obtain a desired fluidity, moreover, the aforesaid side-effects become more obvious. Also, the release agent separates due to stress and the like. In particular, toners manufactured by pulverization have a narrow molecular weight distribution and the brittle release agent easily becomes a pulverization interface, so the surface of the release agent became exposed which was frequently found in fine powder. Therefore, making the toner particles finer to obtain high image quality imposes a stricter requirement on filming.
In order to resolve these issues, a toner having small particle diameters and a narrow distribution of particle diameters is ideal. However, conventional pulverized toners are manufactured by melt mixing a colorant, a charge control agent, an anti-offset agent, and the like in a thermoplastic resin; uniformly dispersing the mixed composition; pulverizing the composition; and classifying the pulverized composition. With such toner manufacturing process, the distribution of toner particle diameters is likely to be wide. Therefore, if one intends to obtain a reproduced image having a good resolution and tone, he should, for example, remove small particles having diameters of 5 μm or less and large particles having diameters of 20 μm or more by classification, resulting in very low yield, which is a drawback. Particularly with color toners, it is difficult for the pulverizing method to uniformly disperse a colorant, charge control agent, and the like in a thermoplastic resin. Uneven dispersion of these components and agents adversely affects the fluidity, developing properties, durability, image quality, and the like of the toner.
Recently, toner manufacturing processes using polymerization has been suggested and put into practice to overcome these problems with the pulverizing method. Polymerization can omit conventional manufacturing steps of pulverizing and kneading, and its low energy consumption, reduction of production time, improvement of yields of manufacturing steps, and the like largely contribute to cost reduction. Moreover, it is easy to make toner particles small and at the same time set the distribution of particle sizes narrower than pulverization method, contributing also to improving quality. Well known techniques include, for example, suspension polymerization, emulsion polymerization, polymer suspension method, and the like.
In suspension polymerization, polymerization-capable monomers, a polymerization initiator, and toner composition materials such as colorants are suspended in an aqueous medium in which a dispersant exists, and then polymerized to obtain toner particles. The problem of this method is that the raw material which can be used is limited to styrene-acrylic resin and therefore polyester, which is suitable for full-color toners, cannot be used. Another problem is that it is difficult to allow the distribution of molecular weight to have two peaks or to control the distribution precisely to obtain low temperature fixing properties and hot offset resistance properties at the same time. Also, among other problems, insufficient cleaning is likely to occur because the shapes of obtained particles are spherical.
Emulsion polymerization is a method for obtaining toner particles including emulsifying polymerization-capable monomers and a polymerization initiator in a water containing a surfactant, conducting polymerization, and allowing formed particulates to aggregate and cohere. Since the method provides irregularly shaped particles, it is superior to suspension polymerization toners in cleaning properties. However, the emulsion polymerization, just like suspension polymerization, can hardly use polyester and is difficult to control molecular weight. Moreover, a considerable amount of the surfactant remains even after being washed with water not only on the surface but also inside the particles, therefore leading to deteriorating the environmental stability of the charge of the toner and widening the distribution of charges, which result in intolerable background shading on output images. Further, since the remaining surfactant contaminates a photoconductor, charge roller, developing roller, and the like, they cannot exhibit their inherent charging capabilities.
In polymer suspension method, a polymer which is used as a binder resin and toner composition materials are dispersed or dissolved in a volatile solvent such as a low-boiling point organic solvent, then the mixture is emulsified or made into droplets in an aqueous medium containing a dispersant, and then the volatile solvent is removed. The method is therefore an in-water particle manufacturing method which is not accompanied by a polymerization reaction. This polymer suspension method is superior in that it can use polyester, but since it includes a step for dispersing or dissolving the toner composition materials in the solvent, it is not possible to use high-molecular weight and/or cross-linked resins, and the method merely forms particles. Therefore, adjustment of polymerization reaction including molecular weight designing is not possible, and accordingly, fixing properties cannot be controlled sufficiently.